Deep wells such as oil and gas wells are typically drilled by rotary drilling methods. Some such methods are described in Walter, U.S. Pat. No. 4,979,577. Apparatus for rotary drilling typically comprises a suitably constructed derrick. A drill string having a drill bit at its lower end is gripped and turned by a kelly on a rotary table.
During the course of drilling operations, drilling fluid, often called drilling mud, is pumped downwardly through the hollow drill string. The drilling fluid exits the drill string at the drill bit and flows upwardly along the well bore to the surface. The drilling fluid carries away cuttings, such as rock chips.
The drill string is typically suspended from a block and hook arrangement on the derrick. The drill string, comprises a drill pipe, drill collars and may comprise drilling tools, such as reamers and shock tools, with the drill bit being located at the extreme bottom end.
Drilling a deep underground well is an extremely expensive operation. Great cost savings can be achieved if the drilling process can be made more rapid. A large number of factors affect the penetration rate that can be achieved in drilling a well.
Around the late 1940s, it was discovered that drilling efficiency could be improved by equipping the openings in drill bits, which allow escape of drilling fluid with nozzles. The nozzles provide high velocity jets of drilling fluid at the drill bit. This innovation resulted in a dramatic increase in achievable drilling rates. Today, almost all drill bits are equipped with high velocity nozzles to take advantage of this increased efficiency. It is worthwhile to note that between 45–65% of all hydraulic power output from a mud pump is typically used to accelerate the drilling mud in the drill bit nozzles.
The flow rate of drilling fluid affects penetration rates. Rock drill bits drill by forming successive small craters in a rock face as individual drill bit teeth contact the rock face. Once a drill bit tooth has formed a crater, rock chips must be removed from the crater. The amount of drilling fluid necessary to effect proper chip removal depends upon the type of rock formation being drilled and the shape of the crater produced by the drill bit teeth. Maintaining an appropriate flow of drilling fluid is important for maintaining a high penetration rate.
The weight on the drill bit also has a very significant effect on drilling penetration rates. If adequate cleaning of rock chips from the rock face is effected, doubling of the drill bit weight will roughly double the drilling penetration rate (i.e. drilling/penetration rate is typically directly proportional to weight on the drill bit). However, if inadequate cleaning takes place, further increases in the drill bit weight do not cause corresponding increases in penetration rate because rock chips not cleared away are being reground, thus wasting energy. If this situation occurs, one solution is to increase pressure and flow of the drilling fluid in an attempt to effect better clearing of rock chips from the vicinity of the drill bit.
Further information on rotary drilling and penetration rate may be found in standard texts on the subject, such as Preston L. Moore's Drilling Practices Manual, published by PennWell Publishing Company (Tulsa, Okla.).
Downhole vibrating tools known as mud hammers have been developed in an effort to increase drilling penetration rates. A typical mud hammer comprises a striker hammer which is caused to repeatedly apply sharp blows to an anvil. The sharp blows are transmitted, through the drill bit to the teeth of the drill bit. This has been found to increase drilling penetration rates. Mud hammers are expensive to operate as drill bit life is significantly reduced by the use of a mud hammer.
In another effort to increase drilling penetration rates of drill strings has yielded various downhole devices which exploit the water hammer effect to create pulsations in the flow of drilling mud. Such devices tend to enhance the hydraulic action of the drilling fluid. Their use has a positive effect on rock chip removal and, consequently, drilling penetration rates. Another effect of these devices is to induce vibrations in the drill string, more specifically in the drill bit itself. This too has a positive effect on drilling penetration rates. Examples of such devices can be found in U.S. Pat. No. 4,819,745 (Walter), U.S. Pat. No. 4,830,122 (Walter), U.S. Pat. No. 4,979,577 (Walter), U.S. Pat. No. 5,009,272 (Walter) and U.S. Pat. No. 5,190,114 (Walter).
While the devices described in these patents have proven to be effective at increasing drilling penetration rates they have a number of disadvantages which has prevented their widespread adoption. It is difficult to design such a tool which will operate reliably under the constantly changing properties of drilling mud and the constantly increasing hydrostatic pressure at downhole locations. This problem is exacerbated by the small space within which downhole tools must fit. In many drilling situations the downhole tools have an outside diameter of only 4¾ inches. Space constraints impose onerous constraints on the design of such tools. Other problems with these devices include:                Downhole conditions are harsh. Operating parts of these tools may not withstand downhole operating conditions for extended periods of time.        Operating parameters cannot be adjusted while drilling is ongoing. This makes it difficult to optimize the performance of these tools.        It is not possible to switch these tools on or off while drilling. This makes it difficult to ascertain the effectiveness of the tools since there is a significant variation in drilling penetration rates from well-to-well even if all drilling parameters are kept constant.        During drilling, these tools are only accessible for repair when they are brought to the surface.        
Despite the significant progress that has been made in underground drilling technology over the past century there remains a need for drilling methods and apparatus which provide increased drilling penetration rates.